linux-kernel - Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in xsave_user/xrestore

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Date:	Mon, 16 Mar 2015 15:36:05 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in
 xsave_user/xrestore_user

On Sun, Mar 15, 2015 at 05:49:48PM +0100, Oleg Nesterov wrote:
> Hello.
> 
> Another a bit off-topic change, but I'd like to finish the discussion
> with Quentin.
> 
> And almost cosmetic. But I added the RFC tag to make it clear that this
> needs a review from someone who understands gcc-asm better. In particular
> I am worried if that dummy "=m" (*buf) is actually correct.
> 
> 
> And I agree with Quentin, user_insn/check_insn can be improved to allow
> clobbers, more flexible "output", etc. But imo they already can make this
> code look a bit better, and "xstate_fault" must die eventually.

FWIW, I did poke at that but there's still something wrong with my macros, will
take a look when I get a chance:

---
 arch/x86/include/asm/xsave.h | 130 +++++++++++++++++++++++++++----------------
 1 file changed, 82 insertions(+), 48 deletions(-)

diff --git a/arch/x86/include/asm/xsave.h b/arch/x86/include/asm/xsave.h
index c9a6d68b8d62..a35e9d49843d 100644
--- a/arch/x86/include/asm/xsave.h
+++ b/arch/x86/include/asm/xsave.h
@@ -67,6 +67,51 @@ extern int init_fpu(struct task_struct *child);
 			_ASM_EXTABLE(1b, 3b)		\
 			: [err] "=r" (err)
 
+#define XSTATE_OP(op, st, lmask, hmask, err)				\
+	asm volatile("1:" op "\n\t"					\
+		     "2:\n\t"						\
+		     ".pushsection .fixup,\"ax\"\n\t"			\
+		     "3: movl $-1,%[err]\n\t"				\
+		     "jmp 2b\n\t"					\
+		     ".popsection\n\t"					\
+		     _ASM_EXTABLE(1b, 3b)				\
+		     : [err] "=r" (err)					\
+		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
+		     : "memory")
+
+/*
+ * 661 and alt_end_marker labels below are defined in ALTERNATIVE*
+ * and we're reusing  them here so as not to clutter this macro
+ * unnecessarily.
+ */
+#define XSTATE_XSAVE(st, lmask, hmask, err)				\
+	asm volatile(ALTERNATIVE_2(XSAVE,				\
+				   XSAVEOPT, X86_FEATURE_XSAVEOPT,	\
+				   XSAVES,   X86_FEATURE_XSAVES)	\
+		     "\n"						\
+		     ".pushsection .fixup,\"ax\"\n"			\
+		     "3: movl $-1, %[err]\n"				\
+		     "jmp " alt_end_marker "b\n"			\
+		     ".popsection\n"					\
+		     _ASM_EXTABLE(661b, 3b)				\
+		     : [err] "=r" (err)					\
+		     : "D" (st), "a" (lmask), "d" (hmask)		\
+		     : "memory")
+
+#define XSTATE_XRESTORE(st, lmask, hmask, err)				\
+	asm volatile(ALTERNATIVE(XRSTOR,				\
+				 XRSTORS, X86_FEATURE_XSAVES)		\
+		     "\n"						\
+		     ".pushsection .fixup,\"ax\"\n"			\
+		     "3: movl $-1, %[err]\n"				\
+		     "jmp 663b\n"					\
+		     ".popsection\n"					\
+		     _ASM_EXTABLE(661b, 3b)				\
+		     : [err] "=r" (err)					\
+		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
+		     : "memory")
+
+
 /*
  * This function is called only during boot time when x86 caps are not set
  * up and alternative can not be used yet.
@@ -77,20 +122,11 @@ static inline int xsave_state_booting(struct xsave_struct *fx, u64 mask)
 	u32 hmask = mask >> 32;
 	int err = 0;
 
-	WARN_ON(system_state != SYSTEM_BOOTING);
-
-	if (boot_cpu_has(X86_FEATURE_XSAVES))
-		asm volatile("1:"XSAVES"\n\t"
-			"2:\n\t"
-			     xstate_fault
-			: "D" (fx), "m" (*fx), "a" (lmask), "d" (hmask)
-			:   "memory");
+	if (static_cpu_has_safe(X86_FEATURE_XSAVES))
+		XSTATE_OP(XSAVES, fx, lmask, hmask, err);
 	else
-		asm volatile("1:"XSAVE"\n\t"
-			"2:\n\t"
-			     xstate_fault
-			: "D" (fx), "m" (*fx), "a" (lmask), "d" (hmask)
-			:   "memory");
+		XSTATE_OP(XSAVE, fx, lmask, hmask, err);
+
 	return err;
 }
...

To be continued...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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